A fight underwater follows these rules.
Impeded Weapons
When making a melee attack roll with a weapon underwater, a creature that lacks a Swim Speed has Disadvantage on the attack roll unless the weapon deals Piercing damage. A ranged attack roll with a weapon underwater automatically misses a target beyond the weapon’s normal range, and the attack roll has Disadvantage against a target within normal range.
Fire Resistance
Anything underwater has Resistance to Fire damage (explained in “Damage and Healing”).